Despite the recent advances in the plant transformation systems, the proportion of totipotent cell that become transformed is very low as compared with non transformed cells. Therefore, purpose of an effective selectable marker, antibiotic or herbicide resistant gene is essential to simplify' the detection procedures in order to pick out putatively transformed plants (Leyman et al., 2004; Mild and McHugh, 2004). In general the existing selection systems can be divided mainly into two groups; the conventional selection systems constitute largest group and relay on an antibiotic or herbicide selective agent being detoxified by selective gene. The other group comprises positive selection systems where selective agent is converted into a simple compound by the selective gene product and transformed cells experience metabolic or developmental advantages (Erikson et al., 2004; Wenck and Hansen, 2005).
Currently more than fifty different selection systems have been reported but only few have been popular and reached practical application. Examples are those that use npt II, hpt and bar genes to develop the first generation of transgenic crops regardless of the tissue systems (Haldrup et al., 1998a; Mild and McHugh, 2004), confers antibiotic resistance along with the gene of interest. However, these techniques still need to become more precise to avoid the inadvertent introduction of undesirable genes, such as those carrying allergenicity or those that can cause weediness and endanger natural ecosystems (Daniell et al. 2001b).
In the recent past, alternatively new less controversial selection systems have been developed which no longer result in the presence of resistance genes in the transgenic plants and require nontoxic selective chemicals, such as xylA, galT and dsdA genes mediating selection on xylose, galactose and D-serine, respectively, and providing the transgenic shoots with a metabolic ascendancy over non-transgenic shoots (Haldrup et al., 1998a; Erikson et al., 2005). Although these systems provide a convenient new selection strategy, they do rely on performance of transgenic tissue/shoots. Moreover, most of these marker genes are bacterial in origin and introduction of such genes into foodstuffs raise ethical issues and has caused unforeseen apprehension among the public (Leyman et al., 2004; Erikson et al., 2005). The recovery of marker-free plants with out the necessity sexual crossing is certainly an advantage, however, complete removal of the markers has hindered swift acceptance of these methods (Breitler et al., 2004; Miki and McHugh, 2004). Recently, in potato demonstrated transformation devoid of selectable marker gene (de Vetten et al., 2003), however selection procedure is tedious and consists of many PCR reactions.
Development of an environmentally friendly marker assisted selection system involving natural plant material is gaining momentum. The overexpression of Arabidopsis Trehalose-6-phosphate synthase (AtTPS1) gene in Arabidopsis and tobacco demonstrated the potential application of this gene in plant transformation (Avonce et al., 2004; Leyman et al., 2004; Leyman et al., 2005).
Tryptophan (Trp), one of the essential amino acid present in the plants, is not synthesized by animals and is the major contributor the indole ring for the synthesis of auxins, glucosinolates, nicotinic acid, phytoalexins and alkaloids (Pruitt and Last, 1993). Biosynthesis of Trp in plants is not constitutive and hence it is produced as and when required. Although the tryptophan biosynthetic pathway is primarily derived from bacteria and fungi, the real biochemistry and mechanism of Trp synthesis and its regulation in plant became well understood only after the innovation of Arabidopsis mutants trp1-1, trp2-1 (Last et al., 1991; Pruitt and Last, 1993) and yucca (Zhao et al., 2001) and yeast cDNA screening (Hull et al., 2000). Two genes, Arabidopsis thaliana tryptophan synthase beta 1 (AtTSB1 and AtTSB2, encode the tryptophan beta subunit in Arabidopsis. Even though both are highly conserved, AtTSB1 mRNA is more abundant than that of AtTSB2 in the leaf tissues (Pruitt and Last, 1993). Hence, the AtTSB1 gene in plants plays a key role in Trp biosynthesis pathway in converting the indole and serine into Trp (Last et al., 1991). Some of the genes that encode certain key enzymes involved in Trp biosynthetic pathway, such as Anthranilate synthase gene (ASA2), in forage legume Astragalus sinicus (Cho et al., 2000) and QASA 1 and OASA2 in rice have been characterized (Tozawa et al., 2001; Kanno et al., 2004).
Recent studies showed that overexpression of tobacco feedback-insensitive ASA2 gene in soybean and tobacco increased free Trp levels in transgenic plants and displayed resistance to the toxic Trp analog 5-methyl-tryptophan (5MT) and α-methyltryptophan (αMT), respectively (Cho et al., 2004; Tsai et al., 2005). 5MT binds specifically to an allosteric site on Anthranilate synthase (ASA) catalytic α-subunit and causes interference in cellular Trp synthesis (Zhao et al., 2001; Cho et al., 2004).